A thermal switch connects or disconnects an electrical load in response to a temperature sensed by the switch. In a typical thermo-mechanical switch found in the prior art, the switching function is performed by electrical contacts that are caused to operate as a consequence of movement of mechanical members, which mechanical members move in response to the temperature being sensed. In such a typical prior art thermo-mechanical switch, the movement of mechanical members is obtained by using a bimetal cantilever beam, a bimetal disc, paraffin, a contained fluid, a rod in a tube, or many other mechanisms that produce a motive force of a member as a consequence of thermal expansion characteristics of materials or as a consequence of differences in the thermal expansion characteristics of materials.
FIG. 1 shows thermo-mechanical switch 100 which is the type of switch described above. Mechanical moving members are contained in temperature-sensing probe 110 which is disposed in hostile environment 160 and remaining portions 120 of switch 100, including connections to an electrical load being controlled (not shown), remain in a more benign environment behind bulkhead 150.
Thermal switching can be performed without recourse to thermo-mechanical means by using electronic circuitry. For example, it is well known to those of ordinary skill in the art that thermistors can be utilized to sense temperature and that electronic components can be utilized to control switching. However, electronic components generally have temperature limitations. For example, most electronic components that are commonly available for use in designing circuits can only operate at temperatures below 125.degree. C. Further, even though many commonly available electronic components can operate at temperatures at about 125.degree. C., only a few electronic components can operate at temperatures at about 150.degree. C. These facts pose serious technical problems in utilizing electronic circuits to fabricate bulkhead-mounted thermal switches. Further, this problem is compounded by the need to dissipate power in the output components of the switch as the electrical load being switched increases in current rating.
A further, but separate, problem that is encountered in designing electronic, thermal switches relates to a desire to have a switch that responses rapidly to changes in temperature, which desire is satisfied by utilizing electronic, temperature-sensing components having small thermal capacity. The problem arises as a consequence of the fact that devices which are small enough to provide a minimum response time are frequently too fragile structurally for easy application in a bulkhead-mounted thermal switch.
A still further problem that is encountered in designing electronic, thermal switches is the problem of providing isolation from electrical noise and temperature stress for electronic measuring circuitry that is sensitive thereto. In particular, the need to include output-power switching circuits in the same package as temperature-measuring circuits imposes a severe isolation requirement on a design. This problem is further complicated by a desire to have the package retain a cylindrical form which is typical of bulkhead-mounted thermal switch packages.
Lastly, a yet still further problem that is encountered in designing electronic, thermal switches arises as a consequence of a need to obtain maximum thermal conductivity between an environment that is being monitored for temperature and electronic, temperature-sensing components while providing maximum electrical isolation of the electronic, temperature-sensing components from the thermal switch package. Maximum thermal conductivity is required to avoid creating a thermal gradient between the environment that is being monitored and the temperature-sensing components. This is required because any such temperature gradient would cause the switch operation to be inaccurate. Further, electrical isolation is required because changes in the measuring circuit as a result of current leakage through packaging materials could seriously affect thermal switch performance and this is especially important if such current leakage changed with time.
As a result of the above, there is a need in the art for a package for electronic components in a switch which dissipates power, provides mechanical protection from shock and vibration, provides isolation between sensing circuits and power switching circuits, provides connectivity between sensing circuits and the environment, and provides electrical isolation between sensing circuits components and the packaging. In particular, there is a need in the art for such packaging for use in fabricating a thermal switch, which packaging dissipates power so that generally available electronic components can be used to design such a switch, which packaging provides mechanical protection from shock and vibration, which packaging provides isolation between temperature sensing circuits and power switching circuits, provides thermal conductivity between temperature-sensing circuits and the environment, and which packaging provides electrical isolation between temperature-sensing circuits and the packaging.